This invention relates to woven papermakers' fabrics and especially to forming fabrics, including those known as fourdrinier belts or fourdrinier wires.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock" is fed onto the top of the upper run of a traveling endless belt. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the machine side of the fabric. In the conventional fourdrinier machine, the forming fabric also serves as a drive belt. Accordingly, the machine direction yarns are subjected to considerable tensile stress and, for this reason, are sometimes referred to as the load-bearing yarns.
Such papermakers' fabrics are manufactured in two basic ways to form an endless belt. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form the endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross-machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross-machine direction and the filling yarns extend in the machine direction. As used herein the terms "machine direction" and "cross-machine direction" refer respectively to a direction equivalent to the direction of travel of the papermakers' fabric on the papermaking machine and a direction transverse this direction of travel.
Effective sheet support and lack of wire marking are important considerations in papermaking, especially in the formation of the wet web. The problem of wire marking is particularly acute in the formation of fine paper grades where the smoothness of the sheet side surface of the forming fabric is critical as it affects paper properties such as sheet mark, porosity, see-through, pinholing and the like. Accordingly, paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing and like grades of fine paper have heretofore been formed on very fine woven forming fabrics or fine wire mesh forming fabrics. Such forming fabrics, however, are delicate, lack stability in the machine and cross-machine directions, and are characterized by relatively short service lives. U.S. Pat. No. 4,564,052 to Borel describes a dual layer fabric having a double float in the machine direction yarns and planar support for the paper stock provided by directly adjacent machine direction wires, rather than the cross-machine direction wires, which are guided parallely through the weft wires to reduce wire marking. The construction of this fabric, however, does not enhance abrasion resistance or wear of the fabric.
It is known to use coarser and stronger fabrics for good service life, which also have reduced amplitude of sheet side knuckles, as in the fabric described in U.S. Pat. No. 4,239,065 to Trokhan.
Similarly, fabrics are known which employ a number of different approaches to improvement of sheet support. Fabrics are frequently inverted to take advantage of the fiber support orientation of the cross-machine direction yarns. Sheet forming on the cross-machine direction yarns does not directly block the smallest of the drainage holes, those which exist between machine direction yarns, and therefore, the fabric drains better and performance improves. Unfortunately, the cross-machine direction yarns are the most widely spaced yarns, and wire marking increases. In an attempt to improve sheet support yet avoid excessive wire marking, fabrics have been produced with increased picks or ends in the conventional weave patterns. This fabric, however, has a reduced rate of drainage and fabric performance.
In short, in order to ensure good paper quality, the side of the papermakers' fabric which contacts the paper stock should provide high support for stock, preferably in the cross-machine direction because support is already provided in the machine direction. Conversely, the side of the papermakers' fabric which contacts the rollers and machine must be tough and durable; these qualities, however, most often are not compatible with the good drainage and fabric characteristics desired for a papermakers' fabric.
In order to meet both standards, fabrics like the one in U.S. Pat. No. 3,885,603 to Philip H. Slaughter are employed. The papermakers' fabric is produced from two different fabrics, one having the qualities desired in the paper-contacting side and the other with the qualities desired in the roller-ccntacting side, stitched together. This type of papermakers' fabric is commonly called a "triple layer fabric". Alternatively, two layers of fabric can be woven at once by utilizing threads of different sizes or of different materials and another thread to bind them together, as in the fabric described in U.S. Pat. No. 4,041,989 to Johansson et al. This fabric is commonly called a "duplex fabric". The problem with both these papermakers' fabrics, however, has been that the thread which interconnects the two layers forms undesirable knuckles, which degrade the paper quality and snag as the fabric slackens with use.